Sacrificial anodic protection based on zinc plating or zinc alloy plating is the most effective and most economical method for the corrosion protection of iron and steel. As a consequence, galvanized steel sheet accounts for 10 million tons or 10% of Japan's annual raw steel output of 100 million tons. Galvanized steel sheet is widely employed for building materials, automobiles, household electrical appliances, and the like.
In sacrificial anodic protection by zinc, the two metals (zinc and iron or steel) are in contact and form an electrochemical cell, and the zinc, as the baser metal, becomes the anode and renders the iron cathodic. This inhibits corrosion of the iron or steel by preventing the anodic dissolution which would occur in the case of iron by itself due to local cell formation. Accordingly, when the zinc in contact with the iron or steel has finally been consumed, the anticorrosion activity disappears. Therefore, preventing corrosion of the zinc layer itself (generally white rust corrosion) is crucial for extending the durability of galvanized steel material, and to this end galvanized steel is normally chromated as an undercoating treatment and then painted.
However, these two anticorrosion treatments (chromate treatment and painting) suffer from certain problems. Thus, while the chromate treatment of zinc-plated steel sheet or zinc alloy-plated steel sheet results in a very substantial inhibition of white rust development, this treatment can cause the development of black rust (known as "blackening") during storage or transport of the galvanized steel sheet. It has been observed that this phenomenon tends to occur more readily in the case of a skin pass-rolling after zinc plating and more easily when several % aluminum is present in the zinc than for ordinary galvanized steel sheet.
On the other hand, there has been a substantial proliferation in metal materials in the sector concerned with the production of colored galvanized steel sheet (widely employed for roofing and siding) by painting zinc-plated steel sheet and zinc alloy-plated steel sheet, particularly in coil form. Because the available surface treatments with reactive phosphate salts have not been able to respond to these developments, coating-type chromate treatments, which can be applied to many types of materials, tend to be used for surface treatment. However, adherence by the paint film is a normal problem here, and a problematic paint film adherence is associated with the bending of galvanized steel sheet and particularly with the bending of ultralow-lead galvanized steel sheet and zinc/aluminum alloy-plated steel sheet.
Flash treatment with, e.g., Ni, Co, Fe, etc., as described, e.g., in Japanese Patent Publication Numbers 52-22,618 [22,618/77]and 52-43,171 [43,171/77]and Japanese Patent Application Laid Open [Kokai or Unexamined]Number 59-177,381 [177,381/84]) is considered at present to be a powerful countermeasure against both blackening after chromating and low paint adherence of painted galvanized steel sheet. Japanese Patent Publication Numbers 52-22,618 and 52-43,171 teach that an improved paint adherence is obtained through a substitutional plating treatment (executed prior to chromating) with an acidic solution (pH about 1.5) containing, e g., Ni.sup.2+, Co.sup.2+, Fe.sup.2+, Fe.sup.3+. In the examples provided therein, the substitutional plating treatment is immediately followed by a water rinse.
Japanese Patent Application Laid Open Number 59-177,381 teaches that blackening can be prevented through the treatment (executed prior to chromating) of zinc-plated or zinc alloy-plated steel sheet with a solution (pH=1-4 or 11-13.5) which contains Ni.sup.2+ or Co.sup.2+. In the examples provided for this particular method, treatment with the Ni- or Co-containing solution is followed by rinsing with tap water. On the subject of the activity of the Ni or Co, it is hypothesized in this reference that black rusting is suppressed by their deposition as the metal or oxide.
Thus, as discussed above, it is already known that a pre-chromating flash treatment of the surface of zinc plating or zinc alloy plating with a transition metal such as Co, Fe, etc. will prevent black rusting on the chromated zinc plate or zinc alloy plate and improve the paint adherence to zinc-plated and zinc alloy-plated steel sheet.
The mechanisms by which the aforesaid Ni, Co, or Fe flash treatment inhibit the blackening of zinc-plated or zinc alloy-plated steel sheet and increase the paint/steel sheet bonding have not yet been established with certainty. However, according to the report on pages 150 to 151 of the Collected Summary of the 60th Scientific General Lecture Meeting of Kinzoku Hyomen Gijutsu Kyokai [The Metal Finishing Society of Japan], there is some type of interaction between the flashed metal and the chromium compound(s). This conclusion was drawn based on the observations that a large amount of flashed metal precipitates at the grain boundaries of the zinc crystals and that the chromium provided by the follow-on coating-type chromate treatment is, like the flashed metal, distributed on the grain boundaries. It is conjectured in this reference that the chromium compound is adsorptively immobilized or fixed on the flashed metal.
Zinc black rust, like white rust, is thought to be a basic zinc carbonate as expressed by the chemical formula (ZnCO.sub.3).sub.x.[Zn(OH).sub.2 ].sub.y, but it differs from white rust as a consequence of a stoichiometric oxygen deficiency. Accordingly, black rust is a corrosion product which develops under oxygen-deficient circumstances, and, in particular, it is thought to be formed accompanying the development of corrosion from the grain boundaries. Due to this, it is possible that the chromium compound (concentrated at the grain boundaries by the flashed metal) contributes to the inhibition of black rust development by inhibiting corrosion from the grain boundaries.
As to why flash treatment with a transition metal, e.g., Ni, Co, Fe, improves the paint adherence of galvanized steel sheet, it is thought that the interaction between the flashed metal and chromium compound(s) serves adsorptively to fix or immobilize the chromium compound(s) on the zinc-plate surface. This strengthens the bonding forces between the zinc-plated surface of the material and the chromium compound(s), with the result that the zinc-plate surface/chromium compound interfacial bonding strength is improved. This particular interface is considered to be the weakest of all the interfaces between the various layers in a painted zinc surfaced object with a chromate undercoating.
As explained hereinbefore, a pre-chromating flash treatment with Ni, Co, Fe, etc., is a powerful countermeasure against both of the major problems associated with galvanized steel sheet (blackening and unsatisfactory paint adherence). Nevertheless, when a flash treatment is implemented prior to chromating, it is known that black rusting is in fact inhibited but that white rust tends to appear rather readily. In addition, although this flash treatment does improve the paint adherence to colored galvanized steel sheet during bending, it is known that it reduces the corrosion resistance of the painted sheet and particularly the corrosion resistance of the back-coat surface (service coat).